Inhibition of uranium metal corrosion



June17,1969

WEIGHT CHANGE (mgslin 'r. P. SPRAGUE INHIBITION OF URANIUM METALCORROSION Filed Oct. 20, 1965 F do P M HPo -2.O gms/l TE A- 3.5gms/l 45N HPo 2.0 gms/l -20 j 0 5 2O 3O EXPOSURE TIME (days) Fig.1

225ppm 0 EXPOSURE TIME (days) 0 INVENTOR. F 2 Theodore P. Sprague BY m 4W ATTORNEY.

United States Patent US. Cl. 1486.15 2 Claims ABSTRACT OF THE DISCLOSUREUranium corrosion problems occurring during the machining of uraniummetal were found to be due to the presence of chloride ions in themachining coolant. This corrosion was effectively inhibited by utilizinga machining coolant consisting of water and about 3 to grams of anorthophosphate compound per liter of water.

The invention described herein was made in the course of, or under, acontract with the US. Atomic Energy Commission. This invention relatesgenerally to coolants for use in machining uranium metal, and moreparticularly to improved aqueous coolants for such use whereby corrosionof the uranium metal due to the presence of chloride ions in the coolantis inhibited.

In machining uranium metal, the selection of a proper coolant is animportant consideration from various standpoints such as, for example,tool wear, ease of metal working, production rates, health hazards, andminimization of fire hazards due to the inflammability of uraniumturnings, an inherent danger in machining uranium. Significantadvancements have been made in coolant technology to where trueWater-solution coolants, e.g., sodium nitritetriethanolamine, have inrecent times efiectively replaced the previously used water base oilemulsion type coolant due to the increased production rates attainablewith water solution coolants While at the same time providing greatlyreduced fire and health hazards. However, these water solution coolantssuffer a significant shortcoming or drawback in that uranium metal whichhad been recently machined showed signs of excessive surface pitting orcorrosion. Upon investigating this problem, it was determined that theuranium metal corrosion was due to the presence of chloride ions in thecoolant solution. These chloride ions may be deposited in an initiallychloride ionfree coolant solution in several different ways with perhapsthe greatest source of chloride ions coming from surface depositsremaining on the metal after it has been annealed in a molten salt bathcontaining chloride. Other possible sources of chloride ions include theatmosphere surrounding the uranium metal working equipment, andperspiration from workers hands. During a typical eight-month uraniummachining period, the chloride ion level in a sodiumnitrite-triethanolamine coolant increased to 80 parts per million(p.p.m.), 38 percent of the nitrite had oxidized to nitrate, and thecoolant had come to equilibrium with the carbon dioxide in the air. Allthese conditions accelerate the corrosion of uranium.

In a uranium metal machining operation using the above-mentionedprevious water coolant, corrosion of the uranium metal began when thechloride content of the coolant attained a level of about 35 p.p.m. Thischloride level was achieved after fusing fresh coolant for about 30 daysbut the attainment of this chloride level can be delayed if the uraniummetal is more thoroughly washed after being annealed in the molten saltbath. Or, if de- 3,450,576 Patented June 17, 1969 sired, the coolant canbe frequently changed to minimize chloride ion buildup in the coolant.However, the more thorough washing of the uranium and the frequentchanging of the coolant add greatly to the cost of machining uranium.

When the chloride concentration reaches about p.p.m. the corrosion ofthe uranium metal begins almost immediately. In other words the greaterthe chloride concentration in the coolant the faster the uranium metalcorrodes; for example, a machined uranium part in a sodiumnitrite-triethanolamine coolant corroded in about one to two weeks at achloride concentration of about 35 p.p.m. and in about one hour at alevel of about 100 p.p.m.

The present invention aims to minimize or obviate the short-comings ordrawbacks of the previous true watersolution cool-ants by providing newwater-solution coolants that are capable of protecting uranium metalfrom corrosion due to the presence of excessive chloride ions in thecoolant. These novel results may be achieved by utilizing a coolantsolution containing an orthophosphate compound or salt to substantiallyincrease the chloride tolerance associated with uranium corrosion. Forexample, a dibasic sodium phosphate coolant solution may increase thechlo ride tolerance about 10 times over a sodium nitrite-triethanolaminetype cool-ant.

An object of the present invention is to provide an improved coolant foruse in machining uranium metal.

Another object of the present invention is to provide true-watersolution coolants that are capable of protecting machine tools, coolantcirculating system, retain the high production rates of the previoustrue water-solution coolants, and allow a higher chloride toleranceassociated with uranium corrosion.

Another object of the present invention is to provide aqueous solutionswith ions of orthophosphate salts to inhibit corrosion of uranium duringthe machining thereof.

A further object of the present invention is to provide uranium metalwith a protective coating to minimize the corrosion of the uranium metalby chloride ions in machining coolants.

A still further object of the present invention is to provideessentially complete protection for uranium metal in a coolant solutioncontaining chloride ions in a concentration of about 100 p.p.m.

Other and further objects of the invention will be obvious upon anunderstanding of the illustrative embodiment about to be described, orwill be indicated in the appended claims, and various advantages notreferred to herein will occur to one skilled in the art upon employmentof the invention in practice.

In the accompanying drawing:

FIG. 1 shows data corresponding to the effect various concentrations oforthophosphates in coolant solutions have upon uranium metal corrosion;and

FIG. 2 shows data corresponding to the extent of uranium metal corrosionat various levels of chloride ion concentration in a dibasic sodiumphosphate coolant sol-ution of the present invention.

The present invention comprises the addition of small quantities ofcertain orthophosphates to a substantially large quantity of water toform solutions particularly adaptable for use as coolants in themachining of uranium metal. As briefly pointed out above, orthophosphateions in uranium machining coolants provide the unique mechanism forsubstantially increasing the tolerance or uranium metal to the corrosiveproperty of chloride ions in the coolants. It is believed that thephosphate ions in the coolant solution protect the uranium metal byproviding the latter with a protective coating. This coating is formedby the reaction of the phosphate ions with the uranium metal andnormally appears as a thin, blue film on exposed surfaces of the uraniummetal. This film has been determined by X-ray diffraction to be U and(UO (PO -4H O. Also, it has been found that this film or coating isself-healing in that, for example, if a surface scratch through the filmon a uranium metal specimen is allowed to oxidize in air and then thespecimen again immersed in the phosphate solution containing chlorideions, the film will reform over the scratch to protect the underlyingmetal from the chloride ions.

Inasmuch as the protective film is formed on the uranium metal by thereaction of the latter with the phosphate ions (PO5 the particularorthophosphate compound or salt utilized to introduce a suitable numberof phosphate ions in the solution is not critical. For example,successful corrosion protection has been obtained with orthophosphatesalts containing a soluble metal such as sodium. Thus, for ease andclarity of description the following discussion of the invention will bedirected primarily to orthophosphates containing sodium, but it is to beunderstood that other orthophosphates containing other soluble metals ororganics, e.g., triethanolamine, may provide the desired corrosioninhibition in a manner substantially similar to orthophosphatescontaining sodium. However, care should be exercised in the selection ofthe orthophosphate compounds or salts to assure that a sufficientquantity of phosphate ions will be introduced in the solution to providethe desired film and that the other constituents of the orthophosphatedo not interfere with the formation of or minimize the effectiveness ofthe film.

While it has been found that orthophosphate com pounds or salts in thecoolant solution provide the unique mechanism of forming the protectivefilm on the uranium metal surface to give complete protection to theuranium metal from corrosion in the presence of chloride ions inconcentrations of about 100 p.p.m. or less, further investigations ofother phosphorous compounds did not prove as fruitful. For example,polysodium metaphosphate and sodium pyrophosphate were examined aspossible coolant solution constituents but were found to providesubstantially less corrosion protection than the orthophosphates, evenat substantially higher concentrations.

The orthophosphate compounds tested include dibasic sodium phosphate (NaHPO and tribasic sodium phosphate (Na PO The solutions of each of thesecompounds may be formed by adding a few grams of a selected compound foreach liter of water used. The particular quantities of the phosphatecompound added to the water for providing adequate protection to uraniummetal from chloride ion induced corrosion has been determined to be atleast 3 grams per liter of water (gm/l.) and preferably about 5 gm./l. Aconcentration of over about 5 gm./l. does not appear to appreciablyenhance the corrosion protection since in a l0-gm./l. solutioncontaining about 100 p.p.m. chloride ions the protection afforded auranium article appeared to be essentially the same as that in a5-gm./l. solution containing a similar concentration of chloride ions.

In order to determine the desired quantities of phosphate in the coolantsolution, aerated dibasic sodium phosphate solutions including chlorideion concentrations of 100 p.p.m. were prepared with different quantitiesof the orthophosphate. As shown in FIG. 1 a uranium specimen immersed ina solution having a phosphate concentration of 2.0 gm./l. showed asubstantial weight change in milligrams per square inch due to corrosionin a short time period whereas uranium in a 3.0-gm./l. solution showedonly a slight weight change after days immersion. On the other hand,uranium immersed in solutions having phosphate concentrations of about5.0 to 10.0 gm./l. showed no weight change or corrosion after two monthsimmersion.

The addition of 3.5 gm./l. of triethanolamine (TEA) to the phosphatesolutions having concentrations of 2.0 and 3.0 gm./l. provided areduction in the amount of weight change in the uranium specimen asshown in FIG. 1. However, a similar addition of triethanolamine to the5.0 to 10.0-gm./1. solution had no effect on corrosion inhibition.

Duplicate tests showing the effect various levels of chloride ionconcentrations have upon uranium immersed in aerated solutionscontaining 5.0 gm./l. of dibasic sodium phosphate are shown in FIG. 2.It was found that a solution containing 225 p.p.m. of chloride ions didnot show uranium corrosion in a two-month period, but that a solutioncontaining 250 p.p.m. chloride ions pitted slightly. Solutionscontaining chloride ions up to 800 p.p.m. resisted attack for severalhours.

The pH of the solutions of the present invention may, in part, influencethe choice of the orthosphosphate compound to be used in the solution.For example, in a tribasic sodium phosphate solution the initial pH wasabout 12.2 with a final pH of about 9.7. This high pH would effectextensive attacks on metals such as aluminum and copper. On the otherhand, a dibasic sodium phosphate solution has an initial pH of about 9.8and a final pH of about 8.4. Thus, with these lower pH values the lattersolution may be preferred. Of course, a quantity of a suitable acid,e.g., nitric acid, could be added to the solutions to further lower thepH values. If the pH of the solution is down to about 7.2, the uraniummay show a slight increase in weight over a prolonged period ofimmersion with the uranium showing a brown color rather than the usualblue.

The corrosive effect of the phosphate-containing solutions of thepresent invention on metals other than the uranium that are expected tobe contacted by the coolant, e.g., the metals in the uranium machiningsystem, are of some importance. Of primary interest is cast iron whichwas found to be attacked significantly by the orthophosphate solutionscontaining chloride ions. It has been found that sodium caprylate inconcentrations of at least about 5 gm./l. of solution may inhibitessentially all corrosion of the cast iron by the coolant. Yellow brassand bronze may be protected from corrosion due to contact with thecoolant by adding about 10 to about p.p.m. of benzo triazole to thephosphate solution. Ferrous metals, except for cast iron, were notattacked by the coolant solution.

Other orthophosphates which may be used to form uranium machiningcoolant solutions of the present invention include triethanolaminephosphate and sodium di- Z-ethylhexylphosphate. Also, while distilledwater may be preferred for forming the coolant solutions, filtered watermay work satisfactorily.

It will be seen that the present invention sets forth new and improvedcoolant solutions particularly suitable for protecting uranium metalduring the machining thereof from corrosion due to the presence ofchloride ions in the coolant solution.

As various changes may be made in the form, construction and arrangementof the parts herein without departing from the spirit and scope of theinvention and without sacrificing any of its advantages, it is to beunderstood that all matter herein is to be interpreted as illustrativeand not in a limiting sense.

I claim:

1. A uranium machining coolant comprising an aqueous solution consistingof water, about 3 to about 10 grams or orthophosphate per liter of waterfor inhibiting corrosion of the uranium when trace quantities ofchloride are in the coolant, 5-7 grams caprylate per liter of water forinhibiting corrosion of cast iron by the coolant, and about 10-8O partsbenzotriazole per million of water for inhibiting corrosion of yellowbrass and bronze.

2. A method of protecting uranium metal from surface corrosion due tothe presence of chloride ions in an aqueous solution used for coolinguranium metal during machining thereof, the method comprising forming athin film of uranium phosphate on exposed surfaces of the uranium metalby contacting the latter with a solution comprising water and phosphateions provided by an orthophosphate in a concentration of about 3 toabout 10 grams per liter of water.

References Cited UNITED STATES PATENTS 6 3,007,871 11/1961 Pardee et a125249.3 X 3,265,620 8/1966 Heiman 25249.3 X

FOREIGN PATENTS 5 644,029 10/ 1950 Great Britain.

OTHER REFERENCES Mayne et a1.: J. Appl. Chem. (London), vol. 10, Oe-

Brown 1486.15 tober 1960, pp. 419-422.

Canzler 1486.15 X Marti 1; 1, 14g 15 RALPH S. KENDALL, Primary Exammer.Snyder et a1. 148-6 15 Talley 252 49,3 US Reamer 25249.3 15 83-169;252-493, 49.9, 387, .394

